filefmt.go

package image

import (
	"bytes"
	"encoding/hex"
	"strconv"

	"github.com/pkg/errors"
	"k8s.io/klog"
)

// MaxExpectedHdrSize defines the Size of buffer used to read file headers.
// Note: this is the size of tar's header. If a larger number is used the tar unarchive operation
//   creates the destination file too large, by the difference between this const and 512.
const MaxExpectedHdrSize = 512

// Headers provides a map for header info, key is file format, eg. "gz" or "tar", value is metadata describing the layout for this hdr
type Headers map[string]Header

var knownHeaders = Headers{
	"gz": Header{
		Format:      "gz",
		magicNumber: []byte{0x1F, 0x8B},
		// TODO: size not in hdr
		SizeOff: 0,
		SizeLen: 0,
	},
	"qcow2": Header{
		Format:      "qcow2",
		magicNumber: []byte{'Q', 'F', 'I', 0xfb},
		mgOffset:    0,
		SizeOff:     24,
		SizeLen:     8,
	},
	"tar": Header{
		Format:      "tar",
		magicNumber: []byte{0x75, 0x73, 0x74, 0x61, 0x72},
		mgOffset:    0x101,
		SizeOff:     124,
		SizeLen:     8,
	},
	"xz": Header{
		Format:      "xz",
		magicNumber: []byte{0xFD, 0x37, 0x7A, 0x58, 0x5A, 0x00},
		// TODO: size not in hdr
		SizeOff: 0,
		SizeLen: 0,
	},
}

// Header represents our parameters for a file format header
type Header struct {
	Format      string
	magicNumber []byte
	mgOffset    int
	SizeOff     int // in bytes
	SizeLen     int // in bytes
}

// CopyKnownHdrs performs a simple map copy since := assignment copies the reference to the map, not contents.
func CopyKnownHdrs() Headers {
	m := make(Headers)
	for k, v := range knownHeaders {
		m[k] = v
	}
	return m
}

// Match performs a check to see if the provided byte slice matches the bytes in our header data
func (h Header) Match(b []byte) bool {
	return bytes.Equal(b[h.mgOffset:h.mgOffset+len(h.magicNumber)], h.magicNumber)
}

// Size uses the Header receiver offset and length fields to extract, from the passed-in file header slice (b),
// the size of the original file. It is not guaranteed that the header is known to cdi and thus 0 may be returned as the size.
func (h Header) Size(b []byte) (int64, error) {
	if h.SizeLen == 0 { // no size is supported in this format's header
		return 0, nil
	}
	s := hex.EncodeToString(b[h.SizeOff : h.SizeOff+h.SizeLen])
	size, err := strconv.ParseInt(s, 16, 64)
	if err != nil {
		return 0, errors.Wrapf(err, "unable to determine original file size from %+v", s)
	}
	klog.V(3).Infof("Size: %q size in bytes (at off %d:%d): %d", h.Format, h.SizeOff, h.SizeOff+h.SizeLen, size)
	return size, nil
}